Climate is defined as the mean and variation of weather over long periods of time (typically, 30 years). I emphasize the “and variation” part because climate change doesn’t just refer to a changes in the mean, it also refers to changes in the variation.
One aspect of that change in variation is highlighted in a new paper (Seneviratne et al. 2014, Nature Climate Change, 4, 161-163) which points out that although global average temperature may have been increasing more slowly recently than in the previous two decades or so, the frequency of hot days in land areas has been increasing faster recently than in the previous two decades or so.
To illustrate the principle, I’ll need daily data, which is perhaps most easily obtained from ECA (European Climate Assessment & Dataset). Here are monthly averages of daily high temperature anomaly from Kremsmuenster, Austria since 1950.
I’ve plotted monthly averages just so the plot won’t be so crowded, but the analysis which follows uses the daily data. The red line (a lowess smooth) shows that there is an upward trend since about 1980, but it was fastest in the 1980s and has been a bit slower since.
I transformed these temperature anomalies to normalized anomalies, by dividing each value by the standard deviation. But I used the standard deviation for the given time of year, so that the seasonal pattern in variation wouldn’t bias the result. Then, I counted how many days out of each year the normalized anomaly exceeded 2.326, which is the 99th percentile for a normal distribution. This doesn’t mean the data necessarily follow the normal distribution, but it is a reasonable cutoff point which should include about 1% of the data in a “normal” year.
And here are the counts for each year:
It too begins to rise about 1980, when the mean high temperature starts its rapid increase. But unlike the mean, this count of extreme hot days doesn’t rise more slowly after 1990 — instead it rises more quickly. In fact its increase is quite dramatic after about 2005, and the years since 2010 have been scorching hot.
We can compare the trends (the smooths) for mean temperature, and for number of hot days, by normalizing them and plotting them on the same graph:
Clearly, although the mean high temperature has risen more slowly since 1990 than it did during the 1980s, the number of hot days has risen faster.
A number of factors could bring about such a change, and one of them is an increase in the variance of daily high temperature. Such an increase is noticeable if we compute the standard deviation of high temperature anomaly, averaged over each year:
The most noticeable change is the increased standard deviation over the last five years, especially the highly deviant year 2013. We see the same thing in the standard deviation of high temperature anomaly averaged over each month:
The smoothed version reveals, among other things, an increase since about 2010:
So, not only has the mean high temperature at this location changed recently, so too has the variation — and both of those are examples of climate change. The increased variation is one of the factors leading to more rapid increase in the number of hot days per year.
I did a similar analysis for all stations in the ECA which had sufficient data for at least 60 years out of the 64-year period from the beginning of 1950 to the end of 2013. Then I averaged the number of hot days over all stations. This is an extremely crude way to estimate the regional effect since it has no area-weighting, but at least it’s a first pass at exploring the phenomenon. Here’s the result:
At first I was a bit surprised that the 2003 European heat wave and the 2010 Moscow heat wave don’t stand out more prominently, in fact the single most prominent “standout” year is 1990. But then I remembered that not only is this a pan-European average, it also includes as “hot days” those days in winter (and spring and fall) which are much hotter than average for those times of year. Although parts of Europe suffered greatly from the heat waves in 2003 and 2010, overall the number of hot days relative to the norm for a given time of year, has shown a more steady increase (excepting 1990) and a more recent increase (there’s no evidence of a slowdown since whatever year the denialists want to cherry-pick).
Seneviratne et al. used a somewhat different approach. They defined “hot days” as those above a given percentile (without regard to the normal distribution), and instead of just counting hot days, they computed what fraction of the land area showed an excessive number of hot days during each year. They also included the entire globe for which data were available. Their overall result, however, is the same: that the number of hot days has increased more rapidly recently in spite of the global average temperature increasing more slowly (I’ve reproduced only part “b” of their figure 1):
They also found that the more extreme one defines “hot days,” the stronger is the most recent increase.
All of which goes to show that even if one defines climate only by the surface air temperature (which I don’t), and even if one accepts that the slowdown in global average temperature is significant (which I don’t), we still have to face the fact that we’ve witnessed climate change over the last decade and a half. It also emphasizes the point that it’s not just the mean that matters, it’s also the extremes. Some would say that the change in extremes is the greater threat to the habitability of our planet.